The T-mobile "web" plan is theoretically only for accessing the web on your phones mobile browser, the $19.99 plan is unrestricted. The cheaper plan doesn't have all ports open, so some things like Sling and VPN won't work.

But in your case you are probably on a Blackberry plan, and there is no alternative for that. Blackberry devices need a special plan to work, they can't just operate on a cheap data plan.

Your $20 is actually pretty cheap, most other operators charge much more for data, especially on a Blackberry.